The present invention relates to techniques for cancelling the interference from interference-corrupted data signals.
Data signals are subject to corruption by various types of interference. For example, phase and/or amplitude distortion in a transmission channel, such as a voiceband telephone channel, can result in intersymbol interference. The latter is conventionally dealt with in state-of-the-art receivers via adaptive feedforward equalization which uses weighted sums of, for example, passband samples of the received data signal to generate a stream of equalizer outputs which are substantially interference-free. The passband equalizer outputs are demodulated to form a stream of baseband outputs in response to which decisions are formed as to what the transmitted data was. An error signal formed in response to the difference between each baseband output and a corresponding decision is used to adaptively update the equalizer weighting coefficients in such a way as to minimize, for example, the average of the square of the error signal.
Adaptive equalizers--or structures that operate like adaptive equalizers--are also used to deal with interference which is not channel-distortion-induced. In adaptive antenna systems used in digital transmission, for example, the interference is in the form of so-called directional noise, or jamming signals. These signals, which impinge on the antenna from directions other than the direction of the desired signal, are effectively removed from the received, interference-corrupted signal by the adaptive equalization, or (more properly in this case) adaptive filtering, process. See, for example, B. Widrow et al., "Adaptive Antenna Systems," Proc. of IEEE, Vol. 55, No. 12, December 1967, pp. 2143-2159.
A disadvantageous property of equalizers of the above-described type--as well as so-called decision feedback equalizers which form weighted combinations of previous data decisions and feed them back to be combined with the received signal--is that at the same time that the distortion in the transmission channel is being compensated for, the noise in the channel, and in circuitry preceding the equalizer, is actually enhanced--at least in those applications in which the distortion is channel-induced. As a result of such noise enhancement, it may be impossible to achieve an acceptably low data error rate in such applications as the transmission of high speed data over channels which have both a high noise level and substantial amplitude (slope) and phase distortion. Transmission of 9600 bit/sec data over switched telephone network channels is a particular example.
One known approach for dealing with the noise-enhancement problem is taught, for example, in U.S. Pat. No. 3,648,171 issued Mar. 7, 1972 to D. Hirsch. There, samples of the received signal are subjected to conventional adaptive equalization and are otherwise processed so as to form decisions--referred to as "preliminary" decisions in Hirsch, but hereinafter referred to as "tentative" decisions--as to what the transmitted data was. The tentative decisions are applied to a processor--referred to in Hirsch as a "final equalizer" but hereinafter referred to as a "canceller"--which forms weighted sums of the tentative decisions. Each such weighted sum--hereinafter referred to as a "cancellation signal"--represents the interference component of a respective received signal sample and is combined therewith. This provides a stream of cancelled samples from which the final data decisions are made. The weighting coefficients used in the canceller are adaptively updated in response to an error signal derived from the difference between each final decision and the corresponding cancelled sample.